1. Field of the Invention
The present invention relates to a semiconductor device and a display device, and more particularly to a technique which is effectively applicable to a display device having a display panel which incorporates a drive circuit therein outside a display region.
2. Background Art
Conventionally, as a TFT element used in a semiconductor device or the like, there has been known a reversely-staggered-type (also referred to as a bottom-gate type) TFT element. The reversely-staggered-type TFT element is a TFT element which is formed by stacking a gate electrode, an insulation film, a semiconductor layer, a source electrode and a drain electrode on a surface of a substrate such as an insulation substrate in this order.
Further, with respect to the reversely-staggered-type TFT element, there has been known a TFT element which arranges the semiconductor layer, the source electrode and the drain electrode on a surface of the insulation film. In such arrangement, a portion of the source electrode and a portion of the drain electrode respectively get over the semiconductor layer. Further, the semiconductor layer includes an active layer, a first contact layer interposed between the active layer and the source electrode, and a second contact layer interposed between the active layer and the drain electrode. Further, the first contact layer and the second contact layer are arranged only on the active layer as viewed from the insulation substrate in general.
The reversely-staggered-type TFT element is used as an active element in an active-matrix-type liquid crystal display panel, for example. The active-matrix-type liquid crystal display panel is a display panel which seals a liquid crystal material between two substrates, wherein on a surface of one substrate out of these two substrates, for example, on a surface of an insulation substrate which possesses high optical transmissivity such as a glass substrate, a plurality of scanning signal lines, a plurality of video signal lines, a plurality of active elements and a plurality of pixel electrodes are arranged. Here, when the active element is a TFT element, such one substrate is referred to as a TFT substrate.
In a liquid crystal display device using the above-mentioned liquid crystal display panel, a control of timing of generation or inputting of scanning signals inputted to the scanning signal lines, and a control of timing of generation or inputting of video signals inputted to the video signal lines are conventionally performed using an IC chip (driver IC) manufactured in a step different from a step for manufacturing the TFT substrate.
However, recently, there has been proposed a liquid crystal display panel which forms an integrated circuit (drive circuit) equivalent to the above-mentioned IC chip outside a display region of the TFT substrate, that is, outside a display region in which active elements and pixel electrodes are arranged in a matrix array in a manufacturing step of the TFT substrate, for example. Hereinafter, the integrated circuit formed outside the display region is referred to as a peripheral circuit.
The peripheral circuit on the TFT substrate is constituted of an n-channel MOSFET (hereinafter referred to as nMOS) and a p-channel MOSFET (hereinafter referred to as pMOS), a CMOS which is formed by combining the nMOS and the pMOS, a resistance element, a capacitive element and the like formed on a surface of a glass substrate, for example. Here, to efficiently manufacture the nMOS and the pMOS, it is desirable that the nMOS and the pMOS have the same constitution as the TFT element arranged in the display region, for example. That is, when the TFT element in the display region is of the reversely-staggered-type, it is desirable that the nMOS and the pMOS which constitute the peripheral circuit also are of the reversely-staggered-type in the same manner as the TFT element.
When the TFT element in the display region of the TFT substrate is of the reversely-staggered-type, the active layer is often formed using amorphous semiconductor such as amorphous silicon (a-Si), for example. Accordingly, in forming the TFT elements in the display region and the TFT element in the peripheral circuit using the reversely-staggered-type structure, to manufacture these TFT elements most efficiently, the active layer of the TFT element of the peripheral circuit may be also formed using amorphous silicon.
However, it is necessary to operate the TFT element in the peripheral circuit at extremely high speed compared to an operation of the TFT element in the display region. Accordingly, it is desirable to form the active layer of the TFT element in the peripheral circuit using poly-crystalline semiconductor such as poly crystalline silicon (poly-Si), for example.
From the above, in manufacturing the TFT substrate in which both of the TFT elements in the display region and the TFT element in the peripheral circuit are of the reversely-staggered-type, in general, for example, an amorphous silicon film used for forming active layers of the respective TFT elements are formed and, thereafter, only a region of the amorphous silicon film where the active layer of the TFT element in the peripheral circuit is formed is formed into polycrystalline silicon.
In forming only the region of the amorphous silicon film where the active layer of the TFT element in the peripheral circuit is formed into polycrystalline silicon, for example, amorphous silicon in the region is dehydrogenated and, thereafter, amorphous silicon is melted by radiation of laser beams, and molten silicon is crystallized or re-crystallized. Further, in forming amorphous silicon into polycrystalline silicon, energy of radiating laser beams and a moving speed of a radiation position are adjusted thus forming polycrystalline silicon which is constituted of a mass of fine crystals or granular crystals mainly formed of strip-shaped crystals elongated in the channel length direction of the formed TFT element, for example. With the formation of such polycrystalline silicon, the mobility of carriers can be enhanced thus realizing the high speed operation of the TFT element.